1. Field of the Invention
The present invention relates to a light-guide plate, and more particularly, to a liquid crystal display device having a light-guide plate for reflecting and diffusing light emitted from a linear light source, such as a white fluorescent lamp or an elongated mini-lamp, disposed at one end of the back of a liquid crystal cell so that the light is uniformly incident on the liquid crystal cell.
2. Description of the Related Art
Conventionally, a display screen of a liquid crystal display device (LCD) requires surface illumination that provides uniform and high brightness and uniform lightness. However, since a white fluorescent lamp, an elongated minilamp, and the like are not surface light sources, but are linear light sources, a light-guide plate is used to reflect and diffuse light from these linear light sources so that the light is uniformly incident on the whole liquid crystal cell.
FIG. 3 is a cross-sectional view schematically showing the structure of a liquid crystal display device using a conventional light-guide plate, FIG. 4 is a cross-sectional view showing the principal part of the liquid crystal display device, and FIG. 5 is a perspective view showing an example of a structure of the conventional light-guide plate.
Referring to FIG. 3, the liquid crystal display device comprises a liquid crystal cell 1, a linear light source 2, such as a white fluorescent lamp or an elongated mini-lamp, disposed at one end of the back side of the liquid crystal cell 1, a light-guide plate 3 disposed on the back side of the liquid crystal cell 1, and a light-diffusing sheet member 4 placed on the light-guide plate 3.
The light-guide plate 3 includes a rectangular plate member 11 made of acrylic resin having a substantially high light transmittance, and has, on its bottom face, multiple concave dots formed longitudinally and laterally for light reflection and diffusion, though they are not shown. Instead of the concave dots, multiple reflecting films may be formed longitudinally and laterally.
An angular-U-shaped guide 12 made of rubber or the like is bonded to the peripheral portion of the upper surface of the plate member 11, excluding one longitudinal end portion, with an adhesive or the like. The height of the guide 12 is set to be slightly larger than the overall thickness of the light-diffusing sheet member 4 placed on the plate member 11.
As shown in FIGS. 4 and 5, the light-diffusing sheet member 4 is formed by sequentially stacking a rectangular diffusing sheet 13, a rectangular lens sheet 14 having pits and projections periodically formed in the lateral direction, and a rectangular lens sheet 15 having pits and projections periodically formed in the lateral direction. The peripheral portions of the sheets 13 to 15 are bonded with doubled-sided adhesive tapes 16 and combined into the light-diffusing sheet member 4, as shown in FIG. 4. The sheets 13 to 15 may be bonded by fusing or by other means instead of using the double-sided adhesive tapes 16. The combined light-diffusing sheet member 4 is fixed on the back of the liquid crystal cell 1 via a spacer 17 with a double-sided adhesive tape or the like.
In this liquid crystal display device, white light emitted from the linear light source 2 enters the light-guide plate 3 from one end face, is reflected and diffused inside the light-guide plate 3 by the multiple dots (or multiple reflecting films), and emerges from the entire upper surface of the light-guide plate 3. The emergent light is diffused and made uniform in the plane direction by the diffusing sheet 13, sequentially passes through the lens sheets 14 and 15, and is thereby turned into light that is uniform in the lateral and longitudinal directions of the sheet. The light enters the liquid crystal cell 1 as surface illumination that is uniformly bright over the plane.
In the above-described light-guide plate 3, the guide 12 made of rubber or the like is bonded to the peripheral portion of the upper surface of the plate member 11 with the adhesive or the like. Therefore, alignment while bonding the guide 12 on the plate member 11 is difficult, and working efficiency is lowered. This is a factor in increasing manufacturing costs.
Since the light-diffusing sheet member 4 is formed by bonding the peripheral portions of the sheets 13 to 15 by the double-sided adhesive tapes 16, the overall thickness thereof increases due to interposition of the double-sided adhesive tapes 16. For this reason, it is difficult to reduce the thickness of the liquid crystal display device, particularly, the thickness of the light-guide plate.
Furthermore, since the peripheral portions of the sheets 13 to 15 are bonded by the double-sided adhesive tapes 16, the center portions thereof are apt to deflect, and the clearances between the center portions of the sheets 13 and 14 and between the center portions of the sheets 14 and 15 differ slightly from the clearance between the peripheral portions. Such deflection and clearance difference cause the appearance of diffraction fringes, so-called called moirxc3xa9 and interference fringes, on the screen of the liquid crystal display device.
The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a simplified and low-profile light-guide plate that does not cause moirxc3xa9 and interference fringes.
In order to achieve the above object, according to an aspect of the present invention, there is provided a light-guide plate, in which light enters a transmissive plate member from one end face, is reflected and diffused inside the plate member, and emerges from one main surface of the plate member. A guide member is formed integrally with the main surface so as to guide a light-diffusing sheet member to be placed thereon.
Preferably, the plate member in the light-guide plate is rectangular, and the guide member is a rib formed along two or three adjoining sides of the main surface of the plate member.
Preferably, the height of the guide member is set to be larger than the thickness of the light-diffusing sheet member to be placed on the main surface.
In the light-guide plate of the present invention, since the guide member guides the light-diffusing sheet member onto the main surface of the plate member, the light-diffusing guide member is easily positioned and fixed merely by being dropped onto the main surface. This eliminates the necessity of fixing the light-diffusing sheet member by using a double-sided adhesive tape as was necessary previously, and the thickness of the overall light-guide plate including the light-diffusing sheet member is reduced, which allows a low profile.
Since the double-sided adhesive tape is not used, clearance does not vary in the light-diffusing sheet member, and moirxc3xa9 and interference fringes are not produced.
Since the guide member is formed integrally with the main surface of the plate member, the structure is simplified, and handling is facilitated. This makes it possible to omit the step of bonding a separate guide member onto one main surface of the plate member in the conventional art, and to thereby reduce manufacturing costs.
When the height of the guide member is set to be larger than the thickness of the light-diffusing sheet member to be placed on the main surface, stress is applied only to the guide member when fixing the light-guide plate, and is not applied to the light-diffusing sheet member to be placed. For this reason, external stress will not be directly applied to the light-diffusing sheet member, and distortion is prevented from being caused due to stress.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.